HVAC systems for use in buildings incorporate electric actuators to control the position of devices such as dampers and valves. In certain applications it would be advantageous to position a valve, open, closed or mid-position by means of an independent actuator in the event of a power failure. This type of valve actuator would be termed fail-safe. Several methods could be used to store the energy required to position the valve in a desired position in the event of a power failure include a mechanical spring, a battery and electric motor combination, or a capacitor and motor combination, which is the subject of the present invention. The use of an electric motor provides the capability to position the actuator open, closed or mid-position and would be easily configurable in the field. Mechanical spring solutions would provide open or closed positioning depending on the design, but would provide no capability for stopping at mid-position or field programmability. A battery would provide the necessary power at the cost of extra complexity. Battery voltage must be monitored to provide an indication if the battery had sufficient capacity to drive the valve to the desired position. Batteries must also be recharged on a continual basis. If the battery was for some reason not functional, a signal would have to be sent to a control panel otherwise the actuator would no longer be deemed fail-safe. In low current draw/standby applications, possibly operating at elevated temperatures, a battery has limited life. The cost of the battery, and lower to replace it, makes the battery solution cost prohibitive.
A better solution would be to store energy in a capacitor. A capacitor has a longer life, is relatively unaffected by temperature, and requires no special charging/monitoring circuit. The drawback would be the size and cost of a capacitor large enough to store the energy required to power the actuator in fail-safe circumstances.